Age old question- Affect or Effect???

I was writing a lab (actually still am) and i didnt know if i should use 'affect' or 'effect'. I soon learned from the following website that i should, in fact, use effect. Now i know what the two mean. Just thought i'd share this useful piece of knowledge...
http://www.wonderfulwritingskillsunhandbook.com/html/affect_vs_effect.html

" Of these three measured quantities, the tare mass of the beaker had the largest effect on the uncertainty of the density calculations."
I think effect is the correct word to use. Now i dont know if it should read "of the density calculations" or "in the density calculations. Normally i wouldnt be so concerned, but this professor was an english minor and he really stresses writing skills at the expense of a lot of red ink!

 

actually it wasnt correct. the uncertainty of the volume had a bigger effect.